Methomyl

Phototransformation in air

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

accepted calculation method

Qualifier:

according to guideline

Guideline:

other: OECD Photochemical Oxidative Degradation in the Environment

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: U.S. EPA Determination of Rates of Reaction in the gas-Phase in the Troposphere §796.3900

Deviations:

no

GLP compliance:

no

Remarks:

OECD Principles of Good Laboratory Practice were not applicable to the preparation of this report.

Specific details on test material used for the study:

- Substance name: Methomyl
- Substance ID: DPX-X1179
- Purity: Not reported

Estimation method (if used):

The accuracy of the estimation methods used was compared against 600 experimentally determined hydroxyl radical rate constants and is within a factor of 2 of the experimental value. An assumed value is a value of a structure fragment estimated by environmental scientists and not derived from experimental values. Structures were entered into the program by SMILES (Simplified Molecular Input Line Entry System).

Group rate constants (kE12 cm³/molecules-sec) are 0.136 for primary carbon and 0.934 for secondary carbon. Substituent factors for hydrogen abstraction are designated as F(X).
Kprim = 0.136 F(-C-) = 0.136 (1.230) = 0.167
Kprim = 0.136 F(-N-C)(=O) O-) = 0.136 (7.500) = 1.020
Kprim = 0.136 F(-S-) = 0.136 (7.800) = 1.061
H Abstraction TOTAL = 2.248E–12 cm³/molecule-sec

In the troposphere, (OH) represents an average concentration which is a function of variables such as season, time of day, temperature, and location. The average 12-hour daylight hydroxyl radical concentration has been estimated as (OH) = 1.5E6 molecules/cm³.

Reference substance:

no

Key result

DT50:

19.307 h

Test condition:

The half–life of the test substance for reaction with average daily air concentrations of hydroxyl radicals (12-hr day; 1.5E6 OH radicals per cm³)

Key result

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Transformation products:

not measured

Conclusions:

The half–life of the test substance for reaction with average daily air concentrations of hydroxyl radicals (12-hr day; 1.5E6 OH radicals per cm³) is 19.307 hours. Hydrogen abstraction and reaction with both nitrogen and sulfur were predicted to contribute to the rate of photochemical degradation.

Executive summary:

The calculation of the second order rate constant and associated half-life for the reaction of test substance in the gas phase in the troposphere, using the method of Atkinson is provided. This calculation is an estimate of the photochemical oxidative degradation (indirect phototransformation) of the test substance. The overall OH rate constant is 6.6481E-12 cm³/molecule-sec. The half–life of the test substance for reaction with average daily air concentrations of hydroxyl radicals (12-hr day; 1.5E6 OH radicals per cm³) is 19.307 hours. Hydrogen abstraction and reaction with both nitrogen and sulfur were predicted to contribute to the rate of photochemical degradation.

Description of key information

Study Type	Study Details	Value	Guideline	Reliability
Phototransformation in Air	OH radicals	DT50 = 19.307 h	OECD Photochemical Oxidative Degradation in the Environment U.S. EPA Determination of Rates of Reaction in the gas-Phase in the Troposphere §796.3900	2

Key value for chemical safety assessment

Half-life in air:

19 h